1. Field of the Invention
The invention plan generally relates to wireless communication systems and, more particularly, to systems for transmitting and receiving signals that utilize frequency plans.
2. Related Art
Wireless communication systems are an integral component of the ongoing technology revolution and are evolving at an exponential rate. Wireless communication systems are generally radio frequency (RF) communication systems. Many wireless communication systems are configured as xe2x80x9ccellularxe2x80x9d systems, in that the geographic area to be covered by the cellular system is divided into a plurality of xe2x80x9ccells.xe2x80x9d Mobile communication devices (e.g., wireless telephones, pagers, personal communications devices, and the like) in the coverage area of a cell communicate with a fixed base station within the cell. The wireless communication system is also capable of communicating with stationary communication devices, though most applications employ the mobile communication devices described above.
In cellular wireless communication systems, the mobile communication devices interface with a base station that is generally a low-power base station. Low-powered based stations are utilized so that frequencies used in one cell can be re-used in cells that are a sufficient distant away to avoid interference. Hence, a mobile communication device user, whether mired in traffic gridlock or attending a meeting, can transmit and receive signals, such as phone calls, so long as the user is within a cell served by a base station.
The communication format used in most wireless communications systems is a high-frequency carrier waveform modulated by low frequency, or xe2x80x9cbaseband,xe2x80x9d signals. The baseband signal may include audio and/or data signals. Mobile communication devices within a wireless communication system typically have a transmitter, the transmitter having a modulator and an upconverter. The modulator xe2x80x9cmodulatesxe2x80x9d the baseband signals (e.g., speech detected by the handset microphone) onto the carrier waveform. The upconverter increases the frequency of the low frequency modulated signals to the carrier waveform frequency appropriate for the wireless communication system. The carrier waveform is then sent from the mobile communication device to a base station. Amplitude modulation (AM) and frequency modulation (FM) techniques, for example, are well known to those of ordinary skill in the art. Mobile communication devices also typically have a receiver, the receiver having a demodulator and a downconverter. The demodulator xe2x80x9cdemodulatesxe2x80x9d a carrier waveform received from a base station to extract a received baseband signal that is then sent for processing to a baseband module of the mobile communication device. The downconverter decreases the carrier waveform frequency to the frequency appropriate for processing by the baseband module of the mobile communication device.
In the mobile communication device, the received carrier waveform and the transmitted carrier waveform are generally processed with a synthesizer-generated signal having a reference frequency. Generally, the synthesizer includes at least two variable controlled oscillators. The oscillators allow the mobile communication device to achieve greater power efficiency by processing the audio and/or data signals at lower frequencies than the carrier frequency. A first variable controlled oscillator may be used to receive audio and/or data, and a second variable controlled oscillator may be used to transmit audio and/or data. Separate variable controlled oscillators may be used for reception and transmission to allow the mobile communication device to operate at more than one carrier frequency. In addition, separate variable controlled oscillators for reception and transmission allows for one to be shut down while the mobile communication device is performing the other function. Furthermore, use of a separate transmit variable controlled oscillators eliminates the necessity for switching variable controlled oscillators in the synthesizer. However, this solution may be more expensive than using a single synthesizer for reception and transmission.
In some communication systems, such as Global System for Mobile Communications (GSM) systems, it is particularly efficient to integrate component functions since transmission and reception are not performed simultaneously. In particular, the value of integrating synthesizer, receiver, and transmitter functions is maximized. However, when functions are integrated, mobile communication devices operating in GSM wireless communication systems are particularly vulnerable to undesirable interactions between signals.
In the transmitter, the carrier waveform that is modulated is usually a high frequency, periodic waveform generated by the synthesizer. The synthesizer may generate the periodic waveform with a variable controlled oscillator. The variable controlled oscillator may be a voltage controlled oscillator. The frequency of the oscillator should be adjustable since the transmitter is often required to transmit on many different frequency channels within a transmission band. In some GSM wireless communication systems, for example extended GSM (EGSM), the transmission band is 880-915 MHz and is divided into 200 kHz channels. Thus, the oscillator frequency must be varied in precise steps of 200 kHz. Voltage controlled oscillators are well suited for such applications since their output frequency is easily adjusted by manipulating a control voltage. However, oscillators producing signals having disparate frequencies produce undesired spurious effects.
Ideally, transceiver synthesizers would only contain one oscillator to eliminate spurious effects. However, because of the widely disparate frequency ranges of the GSM, DCS, and PCS systems, transceivers with a single main oscillator to cover the required frequencies suffer from poor performance characteristics. At the same time, designs employing separate oscillators for each of the bands are undesirable due to the cost involved.
Another problem is that multi-band handsets using multiple synthesizer oscillators utilize off-chip components such as filters for each of the oscillators. The filters may be surface acoustic wave filters. These off-chip components tend to consume excessive space. Thus, they are inconsistent with the goal of providing compact, lightweight, and portable mobile communication devices.
Direct conversion receivers employ an oscillator operating at the same frequency as the received carrier waveform. Direct conversion receivers eliminate the need for some of the off-chip components such as filters. However, current direct conversion receivers are susceptible to self-conversion to DC of the local oscillator signal or large RF blockers. In addition, direct conversion transceivers tend to be vulnerable to leakage between signals on the oscillator frequency and the radio frequency ports of the mixers. A third problem with direct conversion transceivers is that the reference signal tends to leak onto the transmitter components and ends up being radiated by the antenna. This leakage can interfere with other similar receivers that may be in the same area.
The operation of mobile communication devices results in a number of signals with similar frequencies in the same area. This may lead to undesirable interactions between the signals. This problem is particularly acute in non-linear systems such as mixers.
The invention provides a system for transmitting signals using a frequency plan transmitter and receiver. The frequency plan allows for the selection of transmission and reception channels while using a single reference signal for a receiver and a transmitter and avoiding undesirable frequency interactions.
The system for transmitting signals using a frequency plan table can be implemented as follows. A first programmable frequency divider accepts a reference signal having a local oscillation frequency. The reference signal is the product of passing a synthesizer signal through a local oscillation chain. The local oscillation chain is capable of providing a plurality of local oscillation frequencies. The first programmable frequency divider produces a comparison signal having a comparison frequency. A mixer accepts the reference signal and a modulated transmission signal. The mixer produces a transmit-loop signal having an intermediate frequency. A modulator is capable of inserting data into the transmit-loop signal. A second programmable frequency divider accepts the transmit-loop signal and produces a transmit-loop signal having a divided intermediate frequency signal. A phase detector compares the comparison signal and the transmit-loop signal having a divided intermediate frequency. The phase detector produces a signal that controls a variable controlled oscillator. And, the variable controlled oscillator produces the transmission signal.
The operation of the first and second programmable frequency divider, and the synthesizer, may be based on operating parameters stored in a frequency plan table. The frequency plan table operating parameters being based on the desired transmission signal characteristics and the minimization of undesirable frequency based signal interactions within a mobile communication device.
Other systems, methods, features and advantages of the frequency plan will be or will become apparent to one of ordinary skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the frequency plan, and be protected by the accompanying claims.